Where can I find the function definition in atomicity.h?

Where can I find the function definition in atomicity.h?

atomicity.h is part of c++ stl. In its source file, it declares two
functions about atomic operation. Here is the whole source code. Where can
I find the definition of these functions. I don't find it in stl source
code.
#ifndef _GLIBCXX_ATOMICITY_H
#define _GLIBCXX_ATOMICITY_H 1
#include <bits/atomic_word.h>
namespace __gnu_cxx
{
_Atomic_word
__attribute__ ((__unused__))
__exchange_and_add(volatile _Atomic_word* __mem, int __val);
void
__attribute__ ((__unused__))
__atomic_add(volatile _Atomic_word* __mem, int __val);
} // namespace __gnu_cxx